LTE is regarded as “quasi-4G” with Orthogonal Frequency Division Multiplexing (OFDM)/Frequency Division Multiplexing Access (FDMA) at its core. Being good at providing higher data rate, improving cell capacity, reducing a system delay, supporting cell coverage with a maximum radius of 100 km, etc., as compared with 2G/3G technology, LTE is favoured by an increasing number of communication operators, and applies to fields including high-speed train communication and ground-to-air communication, such as an aircraft communications system.
In an LTE physical layer, an uplink synchronous Hybrid Automatic Repeat Request (HARQ) mechanism, where repeated data can be transmitted only on sub-frames with a fixed sub-frame interval, is employed. A downlink HARQ is asynchronous, that is, in each scheduling operation, a base side explicitly tells a terminal side an ID of a HARQ process to be used for flexible scheduling in retransmitting data.
There is a strict timing requirement during HARQ implementation.
FIG. 1 illustrates durations of a radio frame and sub-frames thereof. As can be seen, a radio frame has a duration Tf=10 ms, and includes 10 sub-frames, each having a duration T=1 ms.
FIG. 2 illustrates LTE Frequency Division Duplexing (FDD) uplink HARQ timing. Uplink scheduling information is transmitted at a sub-frame N, N being an integer ranging from 0 to 9. A base station side receives data transmitted by a terminal side at a sub-frame N+4 (i.e., sub-frame with a sub-frame interval of 4). The terminal side will receive check information fed back by the base station side, such as ACK/NACK information, at a sub-frame N+8 (i.e., sub-frame with a sub-frame interval of 8).
FIG. 3 illustrates LTE FDD downlink HARQ timing. A base station side transmits downlink scheduling information at a sub-frame N, receives check information fed back by a terminal side at a sub-frame N+4 (i.e., sub-frame with a sub-frame interval of 4), and can transmit repeated data as early as at a sub-frame N+8 (i.e., sub-frame with a sub-frame interval of 8).
In an LTE system, timing of a terminal side has to be consistent with that of a base station side. To make up for a signal transmission delay, uplink processing at a terminal side has to be performed in advance to ensure consistency in air interface timing. A lead in terminal side uplink processing depends primarily on a distance of a terminal side to a base station side. The greater the distance, the greater the lead.
FIG. 4 illustrates an ideal LTE FDD system (i.e. with no signal transmission delay) with a hardware processing delay of 0.2 ms, i.e., a difference of 0.2 ms between uplink processing timing and downlink processing timing of a terminal side. As can be seen from FIG. 4, time elapsed since the terminal side receives data till processing completes is 3.8 ms.
FIG. 5 illustrates a relationship between LTE FDD uplink processing timing and LTE FDD downlink processing timing of a terminal side with an ultra-long range of 250 km, a hardware processing delay of 0.2 ms, and a signal transmission delay of about 1.66 ms. Thus, an uplink lead of the terminal side is 1.86 ms, while time elapsed since the terminal side receives data till processing completes is reduced to 2.14 ms.
It thus can be seen that, for communication with ultra-long-range coverage (with a radius greater than 100 km), such as in a transoceanic international airline or an airline across an uninhabited desert or forest, to meet a system HARQ processing timing requirement, actual processing time of a terminal side reduces with an increasing propagation distance. Therefore, to cope with a reduced processing time of a terminal side, input cost has to be increased to improve processing capability of the terminal side. However, as the processing time reduces, it is inevitable that processing required will go beyond the processing capability of the terminal side, leading to malfunction of the entire system. In view of this, there is a pressing need for a solution for implementing communication of ultra-long-range coverage using LTE technology.